Position the rack in the middle of the oven and preheat to 350 degrees F.
Put the butter in a microwave-safe bowl, cover and microwave on medium power until melted. Alternatively melt the butter in a small saucepan. Brush some butter on the inside of a 9-by-13-inch pan and cool the remaining butter slightly. Whisk the remaining cooled butter, brown sugar, eggs and vanilla in a large bowl until smooth.
Whisk the flour, baking soda and salt in another bowl. Stir the dry ingredients into the wet ingredients with a spoon or spatula, taking care not to over mix. The batter should be thick and all of the flour mixture should be fully incorporated. Fold in the chocolate chips.
Spread the batter evenly in the prepared pan. Bake until the edges are browned and the center is completely set, about 30 minutes. Cool completely, at least one hour, cut into 24 pieces and serve.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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